mining doesn't cost me more than running my computers already does.
Mining will cost more than having a PC sit idle, always. If you're mining, the CPU or GPU is FULLY utilized and will typically require MUCH more electricity to use than if it were doing nothing or just running a screen saver. Plus there will be significant cooling required, so whatever fans the PC has will likely be going full speed.
Yes, but my PC's never sit idle. If I weren't mining, I'd be running BOINC, or 10s of VMs. So its not much different, since BOINC would be eating all my cores if mining wasn't.